1. Field of the Invention
The present invention generally relates to a color television signal separating filter in a television receiver set, and more particularly, to a color television signal filtering circuit for separating both a luminance signal and a chrominance signal from a composite color television signal or composite color video signal utilizing a NTSC scheme depending upon the correlation of pictures in vertical and horizontal directions.
2. Description of the Prior Art
It is well known that a television receiver set utilizing the NTSC scheme is provided with a color television signal filtering circuit for separating both a luminance signal and a chrominance signal from a televised composite color television signal. For the purpose of discussion of the prior art filtering circuit of the type referred to above, reference will now be made to FIG. 1 of the accompanying drawings.
As shown in FIG. 1, the prior art filtering circuit has an input terminal 1 to which a televised composite color television signal is applied. This composite color television signal is in turn fed through an analog-to-digital converter 4 to a vertical direction filter 6j and also to a front one-line delay circuit 5i. An output from the front one-line delay circuit 5i is supplied to the vertical direction filter 6j both directly and through a another one-line delay circuit 5j which acts to delay the output from the front one-line delay circuit 5i for a length of time corresponding to one scanning line period. The vertical direction filter 6j is generally referred to as a two-line comb filter having its output coupled to a horizontal filter 6k which provides a chrominance signal 205 to an output terminal 2 and also to a first input terminal of a subtracting circuit 9d having its second input terminal adapted to receive an output from the front one-line delay circuit 5i through a compensating delay circuit 5k which is utilized to compensate for the delay occurring in the horizontal filter 6k. The subtracting circuit 9d provides as its output a luminance signal 207 which emerges from an output terminal 3.
The filtering circuit of the above described construction operates in the following manner relative to the composite color television signal based on the NTSC scheme.
The composite color television signal 201 sampled in synchronism with a color subcarrier at a sampling frequency of fs=4.multidot.fsc wherein fsc represents the color subcarrier frequency, when reproduced on a phosphor screen of a cathode ray tube, represents a two dimensional grid pattern as shown in FIG. 2 wherein reference character Y represents the luminance signal and reference characters C1 and C2 represent respective chrominance signals. Specifically, since according to the NTSC scheme the parameter fsc is equal to (455/2).multidot.fH, wherein fH represents the horizontal scanning frequency, the phase of the chrominance signal C for each horizontal scanning line corresponds to the chrominance signal shifted 180.degree. in phase and extracted four samples for each cycle. Let it be assumed that with the use of a Z transform, the one-sample delay and the one-line delay are, respectively, expressed by Z.sup.-1 and Z.sup.-.iota., and the following relationship can be obtained. EQU Z.sup.-1 =exp.multidot.(-j2.pi.f/4.multidot.fsc)
Since fsc=(455/2).multidot.fH, .iota.=910.
The vertical direction filter 6j then operates to process the one-line delayed output from the front one-line delay circuit 5i and the two-line delayed output from the rear one-line delay circuit 5j to extract and provide a line support signal 204, including the chrominance signal, which signal 204 is in turn applied to the horizontal filter 6k. The transfer function Hv(Z) of the vertical direction filter 6j is expressed by the following equation. EQU Hv(Z)=(-1/4).multidot.(1-Z.sup.-.iota.).sup.2
In other words, the vertical direction filter 6j operates to extract the line support signal Hc(m, n) which is expressed by the following equation, wherein (m, n) represent respective points on the axes of abscissas and ordinates on the screen of the cathode ray tube shown in FIG. 2 being taken as the system of coordinates. EQU Hc(m, n)=(-1/4).multidot.[S(m, n-1)-2S(m, n)+S(m, n+1)]
Since the line support signal contains the luminance signal Y, the chrominance signal C(m, n) which is a high frequency component is separated by the horizontal filter 6k from the line support signal Hc(m, n). The chrominance signal 205 so separated from the line support signal is in turn not only outputted through the output terminal 2, but also applied to the subtracting circuit 9d. The subtracting circuit 9d then operates to subtract the chrominance signal C(m, n) 205 from that signal S(m, n) which is a version of the one-line delayed output from the front one-line delay circuit 5i delayed by the compensating delay circuit 5k for a length of time determined by the operating characteristic of the horizontal filter 6k, thereby providing the luminance signal Y(m, n) 207. In other words, the subtracting circuit 9d operates to execute the following equation. EQU Y(m, n)=S(m, n)-C(m, n)
The transfer function Hh(Z) of the horizontal filter 6k may be the one such as expressed by the following equation. EQU Hh(Z)=(-1/32).multidot.(1-Z.sup.-2).sup.2 .multidot.(1+Z.sup.-4).sup.2 .multidot.(1+Z.sup.-8)
The prior art color television signal filtering circuit of the above described construction has some problems. More specifically, the prior art color television signal filtering circuit employs a combination of a vertical direction filter and a horizontal direction filter both having a fixed operating characteristic, such that both of the luminance signal and the chrominance signal in each of the vertical and horizontal directions can be separated from the composite color television signal. According to this prior art method, it has often been observed that, at a region of the reproduced picture where change in both of the brightness and the color is considerable, the luminance signal and the chrominance signal tend to leak into mutual channels resulting in the reproduction of the television picture in which interference of dots occurs. Thus, the prior art color television signal filtering circuit constitutes one of the major causes of deterioration in quality of the reproduced television pictures known as a dot crawl.